I think it is because there is a sense that it doesn't matter whether or not the DM is being unreasonable.
From your position, it doesn't matter what the DM has done or said, they can never be the naughty word. They are never the one who has to compromise their position. The player is the only one who can be an naughty word. I mean, just look at your example you gave me.
I'd… actually cop to that. Yes; when the DM is defining the parameters of the campaign, it's pretty hard to imagine any character creation limitations set by the DM which are inherently
unreasonable. Assuming, that is, that the DM has defined those limitations in vacuo, without player input (which IME is typically how it's done). For the DM to be "the naughty word," they would have to solicit player input and then work to subvert it. If a DM were to ask a group of potential players about what characters they'd like to play and then (through either malice or negligence) create a campaign world that excludes those particular character types, that would be a dick move, without question. But it's such an unrealistic scenario—I can't see any DM acting in good faith ever doing such a thing.
The referee shut down their concept. And there is to be no questioning of that. The why is meaningless to your position. They have spoken, and that is all that matters.
Now all weight of being reasonable falls on the player to alter their position until it is acceptable to the referee, or they leave the game.
Yes. And not just because the DM has more "authority" (however it's been vested or defined) over the setting and the rules than the player; it's because the DM has more
information. The DM by virtue of having created the campaign
knows more about the campaign than a player at character creation possibly can. It might sound paternalistic to say "the DM knows best," but in this case it's literally true.
At what point did you think the player was the problem? What would it take for you to think the DM is the problem?
There are
lots of ways for a DM to be the problem once the game is in full swing. That's kind of orthogonal to the issue of setting limits on character creation.
That is why some of us have a hard time agreeing with you, because some of us have seen this attitude in practice. "I'm not the problem, you are the problem, you are ruining my game and refusing to compromise. I don't need to compromise, I'm the Dungeon Master. If you don't like it, there's the door."
And sometimes, it is the player. I'm not going to say the player is never the problem. But it is like pulling teeth from a rabid lion to get some people to admit that maybe, just maybe, sometimes the DM is the problem.
During play, the DM can be the source of all kinds of problems. That's not in dispute. But what could possibly constitute problem behavior when
defining a setting and its parameters? Subverting the players' intentions preemptively; I've dismissed that as implausible. Creating a naughty word-up setting (e.g. Gor or some such dreck) when the players aren't on board with it would count, I suppose, but again that's not good faith DMing, that's just a bait-and-switch.
Excluding dwarves (or whatever) from a given setting and being non-negotiably strict about it is hardly on the same plane. In fact, I'm having trouble envisioning any scenario (short of the DM deliberately targeting a player they know who e.g. always plays dwarves) where this could constitute "problem DMing." I say again: there is
no meaningful difference between a hypothetical D&D campaign that doesn't allow dwarves and a Cthulhu or Traveller or Vampire campaign that doesn't allow dwarves. And yet you never hear anyone argue, "the ST should be willing to negotiate—maybe make a little room in the setting—insert the Dvergr as a clan neatly between the Tzimisce and the Ventrue!"